Surface Mount FETs compared
One particular electronics component that I often found difficult to pick for my projects is the venerable MOSFET. The FET (Field Effect Transistor) is part of the Transistor family of semiconductors. There are many types of transistors, but they basically fall in one of these three broad categories:
- Current-driven: BJT (Bipolar Junction Transistor)
- Voltage-driven: FET (JFET and MOSFET)
- Other (e.g. phototransistors)
In this overview I focus mostly on Enhancement-mode MOSFETs and present a selection guide with examples.
BJTs and FETs can be used for either switching or amplification, but generally one type performs better than the other.
- BJTs require a constant current flow through the base in order for the transistor to open. This base-driven current flow can easily exceed the current rating of microcontroller ports and additional drivers (BJTs or FETs) are often required to enable higher drive currents. BJTs however outperform FETs in terms of amplification factor. Although BJTs can be used for switching purposes, they are not the most obvious choice when we need to save overall power consumption.
- FETs are voltage driven and can usually be connected directly to microcontroller ports. There is the factor of inrush-current if a FET has a high capacitance, but a simple series current-limiting resistor is sufficient to deal with that. FETs may also need to be selected to be compatible with the logic-level (TTL) voltages and may therefore also require additional drivers to enable higher (or lower) drive voltages. They are however a better choice than BJTs when it comes to low-power switching.
Lastly, before we get to our overview, there is one more distinction I want to address: JFET vs MOSFET. The JFET (Junction-FET) has certain benefits over the MOSFET. JFETs have a simpler production process and are therefore usually cheaper. JFETs also have higher gate input capacitance which makes them more robust to ESD (Electro-Static Discharge) compared to MOSFETs. However, since MOSFETs are the FET of choice for big players like Intel their prices have come down and characteristics have improved. It seems that, like BJTs, JFETs are slowly but steadily giving way to MOSFETs for many applications except where their niche is unchallenged.
(This page is a work in progress)
N-Channel, Enhancement-mode MOSFETs
RrdOn - the higher the difference between Gate and Source the lower the On-Resistance between Drain and Source (Rdson). Therefore, if the Vgsth has a low value it is easier to get a better (lower) on-resistance (which is probably what you want).
P-Channel, Enhancement-mode MOSFETs
|BSH203||-0.68V||-30V||-470mA||0.66R (@ -4.5Vgs, -280mA Id)|
|NDS352AP||-1.7V||-30V||-900mA||0.45R (@ -4.5Vgs, -900mA Id)|
|BSS84||-1.4V (@ -1mA)||-50V||-130mA||12R (@ -4.5Vgs, -100mA Id)|